Ferroelectric materials are characterized primarily by a spontaneous polarization, the orientation of which can be reversed by an electric field. In addition, these materials also display unique dielectric, pyroelectric, piezoelectric and electro-optic properties that are utilized for a variety of applications such as capacitors, dielectric resonators, heat sensors, transducers, actuators, nonvolatile memories, optical waveguides and displays. For device applications, however, it is useful to fabricate ferroelectric materials in the form of thin films so as to exploit these properties and the design flexibility of thin film geometries. Ferroelectric thin films can be deposited using different techniques such as physical vapor deposition, chemical vapor deposition and chemical solution processes including sol-gel and metalorganic decomposition.
Although ferroelectric materials have found a number of applications in several demonstrative ferroelectric devices, the primary impetus of recent activity in ferroelectric thin films is the large demand for commercial nonvolatile memories. The polarization in a ferroelectric shows hysteresis with the applied electric field; at zero field, there are two equally stable states of polarization, + or -P.sub.r, as shown in FIG. 1. This type of behavior enables a binary state device in the form of a ferroelectric capacitor (metal-ferroelectric-metal) that can be reversed electrically. Either of these two states could be encoded as `1` or `0` in a computer memory and since no external field (power) is required to maintain the state of the device, it can be considered a nonvolatile memory device. To switch the state of the device, a threshold field (coercive field) greater than + or -E.sub.c is required. In order to reduce the required applied voltage, the ferroelectric materials need to be processed in the form of thin films. Integration of ferroelectric thin film capacitors into the existing VLSI results in a true nonvolatile random access memory device (see J. F. Scott and C. A. Paz de Araujo, Science, 246, (1989), 1400-1405). In addition to the nonvolatility , ferroelectric random access memories (FRAMS) also offer high switching speeds, low operating voltage (&lt;5 V), wide operating temperature range and high radiation hardness. Furthermore, the ferroelectric thin films, electrodes and passivation layers can be deposited in separate small facilities thereby obviating the need for any changes in the existing on-line Si or GaAs VLSI production. In principle, FRAMS could eventually replace static RAMS (SRAMs) in the cache memory, dynamic RAMs (DRAMs) in the main system memory and electrical erasable programmable read only memories (EEPROMs) in look-up tables.
Although ferroelectric thin films offer great potential for nonvolatile RAMs, commercial usage has been hindered largely by serious degradation problems such as fatigue, leakage current and aging that affect the lifetime of ferroelectric devices. A common source for these degradation properties in oxide ferroelectrics is the presence of defects such as oxygen vacancies in the materials. Considering the problem of fatigue, ferroelectrics are noted to lose some of their polarization as the polarization is reversed. This is known as fatigue degradation and is one of the prime obstacles to forming high quality ferroelectric thin films. The hysteresis loop "shrinks" on account of fatigue and finally after a large number of cycles it is difficult to distinguish between a `1` and a `0` in the memory device, thus rendering it ineffective. Fatigue (see I. K. Yoo and S. B. Desu, Mat. Sci. and Eng., B13, (1992), 319; I. K. Yoo and S. B. Desu, Phys. Stat. Sol., a133, (1992), 565; I. K. Yoo and S. B. Desu, J. Int. Mat. Sys., 4, (1993), 490; S. B. Desu and I. K. Yoo, J. Electrochem. Soc., 140, (1993), L133) occurs because of both the relative movement of oxygen vacancies and their entrapment at the electrode/ferroelectric interface (and/or at the grain boundaries and domain boundaries). These defects are created during the processing of ferroelectric films (with the desired ferroelectric phase). Under an externally applied a.c. field (required to cause polarization reversal), the oxygen vacancies have a tendency to move towards the electrode/ferroelectric interface as a result of the instability of the interface. Eventually, these defects are entrapped at the interface and cause structural damage. This results in a loss of polarization in the material.
There are two possible solutions to overcome fatigue and other degradation problems. The first is to reduce the tendency for entrapment by changing the nature of the electrode/ferroelectric interface. Multilayer electrode structures using ceramic electrodes such as RuO.sub.2 which minimize oxygen vacancy entrapment have been used to minimize fatigue problems in oxide ferroelectrics (see U.S. patent application Ser. No. 08/104,861 filed Aug. 8, 1993). The second solution involves the control of defect density. The extrinsic point defect concentration may be minimized by reduction of impurity concentration or through compensation of impurities. La and Nb doping are known to reduce the fatigue rate of PZT thin films on Pt electrodes by compensating for the vacancies (see S. B. Desu, D. P. Vijay and I. K. Yoo, Mat. Res. Soc. Symp., 335, (1994), 53). The strategies for minimizing the intrinsic defect concentration may include choosing compounds with inherently high defect formation energies or choosing compounds that have no volatile components in their sublattice exhibiting ferroelectric properties. Thus, another alternative to overcome fatigue and other degradation problems is to use a ferroelectric compound that does not contain any volatile components in its sublattice that exhibits ferroelectric properties. This criterion is satisfied by many of the known layered structure ferroelectric oxides.
In the layer-structure family, a large number of compounds of the general form (Bi.sub.2 O.sub.2).sup.2+ (M.sub.n-1 R.sub.n O.sub.3n+1).sup.2-, where M=Ba, Pb, Sr, Bi, K or Na, n=2, 4 or 5 and R=Ti, Nb or Ta, are known to be ferroelectric (see E. C. SubbaRao, J. Phys. Chem. Solids, 23, (1962), 665; B. Aurivillius, Arkiv Kemi 154!, (1949), 463; E. C. SubbaRao, J. Chem. Phys., 34 2!, (1961), 695; G. A. Smolenski, V. A. Isupov and A. I. Agranovskaya, Fiz Tverdogo Tela, 33!, (1961), 895). These compounds have a pseudo-tetragonal symmetry and the structure is comprised of stacking of n perovskite-like units of nominal composition MRO.sub.3 between Bi.sub.2 O.sub.2 layers along the pseudo-tetragonal c-axis. A large number of these compounds do not contain any volatile components in their sublattice that exhibit spontaneous polarization. The tendency for formation of defects such as oxygen vacancies and thereby the degradation problems such as fatigue may thus be reduced.
Novel deposition processes for fabrication of thin film layer structure oxides have been disclosed earlier. However, for ferroelectric device applications, it is also necessary to develop a patterning technology for these devices. Memory devices such as dynamic and random access memories have decreased in size over the last two decades. As the capacity of memory cells has increased and the size has decreased, the design of the cells has become increasingly complex in order to preserve sufficient electrical capacitance to hold the electrical charge representing the data. To produce these complex shaped cells, it is first necessary to transfer the required geometric shape from a mask to the surface of the film to be patterned. This is accomplished by the process of lithography. In a typical lithographic process, a photosensitive polymer (photoresist) film is coated on top of the thin film layers (deposited on a substrate) to be patterned, dried and then exposed with the appropriate geometrical patterns through a photomask to ultraviolet or other radiation. The substrate holding the film layers is then soaked in a solution that develops the images in the photosensitive material. Depending on the type of the polymer used, either exposed or non-exposed areas of the polymer film are removed in the developing process. To produce circuit features, these resist patterns must be transferred into layers comprising the device. A preferred method of transferring the patterns is to selectively remove the unmasked portions of the layers comprising the device. This process is generally known as etching (see S. M. Sze, "VLSI Technology", McGraw-Hill Co., 1983).
Etching techniques can be broadly classified into wet and dry etching processes. Selectivity for wet etching is usally good, most of the time much better than dry etching. The main disadvantage of wet etching is poor line width control and limited resolution because it is essentially an isotropic etch resulting in an undercut of the mask equal to the thickness of the layer to be etched. Wet chemical etching processes are not popular in applications where complex patterning is required because of the low etch rates, poor etch anisotropy, poor uniformity and the poor selectivity that they provide. On the other hand, dry etching techniques are highly suitable for device applications because this patterning technology provide rapid rates of etching of the materials, high resolution so that it can be used in complex configurations, good selectivity so that the underlying materials are not etched when not required, and also provide uniform etching. Dry etching techniques comprise a whole class of etching processes that use plasmas in the form of low-pressure gas discharges to accomplish the etching process. Broadly, this method is comprised of techniques such as sputter etching, ion milling, plasma etching, reactive ion etching and reactive ion beam etching.
In the past, perovskite type ferroelectrics such as PZT and PLT have been etched by techniques such as laser-induced sputtering (see M. Eyett, D. Bauerie, W. Wersing and H. Thomann, J. Appl. Phys, 62, 1987, 1511), chemical wet etching (see H. T. Chung and H. G. Kim, Ferroelectrics, 76, 1987) and reactive ion etching (see M. R. Poor, A. M. Hurt, C. B. Fledermann and A. U. Wu, Mat. Res. Soc. Symp. Proc., 200, 1990). However, it is believed that the etching of layer structure oxides ferroelectrics has never been reported before. This may be because of an inability to identify an etching gas for the plasma that can meet the stringent conditions required for device applications. In the present invention, an etch gas is identified, CHC1FCF.sub.3, for dry etching of ferroelectric layered structure oxide thin films.